Turn on warnings as errors for (most) third_party code on Windows

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 692 matching lines @@
     # but that doesn't work as we'd like.
     'msvs_debug_link_incremental%': '2',
 
     # Needed for some of the largest modules.
     'msvs_debug_link_nonincremental%': '1',
 
     # Turns on Use Library Dependency Inputs for linking chrome.dll on Windows
     # to get incremental linking to be faster in debug builds.
     'incremental_chrome_dll%': '0',
 
+    # The default settings for third party code for treating
+    # warnings-as-errors. Ideally, this would not be required, however there
+    # is some third party code that takes a long time to fix/roll. So, this
+    # flag allows us to have warnings as errors in general to prevent
+    # regressions in most modules, while working on the bits that are
+    # remaining.
+    'win_third_party_warn_as_error%': 'true',
+
     # This is the location of the sandbox binary. Chrome looks for this before
     # running the zygote process. If found, and SUID, it will be used to
     # sandbox the zygote process and, thus, all renderer processes.
     'linux_sandbox_path%': '',
 
     # Set this to true to enable SELinux support.
     'selinux%': 0,
 
     # Clang stuff.
     'clang%': '<(clang)',
@@ skipping 942 matching lines @@
             'defines': [
               '_CRT_SECURE_NO_DEPRECATE',
               '_CRT_NONSTDC_NO_WARNINGS',
               '_CRT_NONSTDC_NO_DEPRECATE',
               '_SCL_SECURE_NO_DEPRECATE',
             ],
             'msvs_disabled_warnings': [4800],
             'msvs_settings': {
               'VCCLCompilerTool': {
                 'WarningLevel': '3',
-                'WarnAsError': 'false', # TODO(maruel): Enable it.
+                'WarnAsError': '<(win_third_party_warn_as_error)',
                 'Detect64BitPortabilityProblems': 'false',
               },
             },
           }],
           # TODO(darin): Unfortunately, some third_party code depends on base/
           [ 'OS=="win" and component=="shared_library"', {
             'msvs_disabled_warnings': [
               4251,  # class 'std::xx' needs to have dll-interface.
             ],
           }],
@@ skipping 1704 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }